The present invention relates generally to digitizing tablets, and more particularly to a method and apparatus for reducing noise in an electrostatic digitizing tablet or panel associated with a computer.
Electrostatic digitizing panels having a resistive layer covered with a non-conductive plate such as glass, are known in the art. The known electrostatic digitizing tablets may operate in one of two modes, namely a touch mode or a pen mode. When operating in the touch mode, a computer is conventionally configured so as to bias the resistive layer with an AC signal. An object such as a user's finger that approaches and is in proximity to the non-conductive plate acts as a load that is capacitively coupled to the resistive layer. The capacitively coupled load causes electric current to flow through the corners of the resistive layer. The computer may determine a Cartesian coordinate (X, Y) position of an object relative to the digitizing panel, based on the current flow in each of the corners of the resistive layer in a manner known to one of ordinary skill in the art. More particularly, the position of the stylus relative to the digitizing panel may be determined based upon a ratio of the corner currents or signals that flow through the corners of the resistive layer due to the object being capacitively coupled to the resistive layer.
When operating in the pen mode, the computer is conventionally configured so as to be receptive to a signal transmitted from a hand-held pen or stylus. In particular, a stylus typically includes a battery portion which supplies power to an oscillator portion for stimulating a coil associated with a transmitter portion to transmit an AC signal from a tip of the stylus. The AC signal may be transmitted from the stylus tip when the tip is in proximity to the resistive layer associated with the digitizing panel. The transmitted AC signal is typically capacitively coupled to the resistive layer associated with the electrostatic digitizing panel. The capacitively coupled AC signal induces an electric current flow through each of the corners of the resistive layer in a known manner. As in the touch mode, the position of the stylus relative to the digitizing panel may then be determined in a known manner. The stylus position determination in the pen mode is based upon a ratio of the corner currents or signals that flow through the corners of the resistive layer due to the AC signal transmitted from the stylus.
One problem with known electrostatic digitizing panels is their susceptibility to the presence of large impulse-type noise sources that may be coupled to the resistive layer. It should be appreciated that the magnitude of the corner signals attributable to an object that is capacitively coupled to the resistive layer is very small compared to external noise sources that couple to the resistive layer. Typically, external noise sources are of two types, namely synchronous noise sources and asynchronous noise sources. Synchronous noise sources may be generated by subassemblies within or adjacent to the computer. For instance, digitizing panels typically overlay a display screen such as an LCD (liquid crystal display) or active matrix screen. The LCD screen generates switching noise which is caused by an M-field or commutating clock that produces a 30-40 Hz square wave for driving the LCD screen. When the square wave changes states, a relatively high (30 volt) signal transient may be coupled onto the resistive layer. On the other hand, asynchronous noise occurs randomly and is attributable to sources such as static electricity, EMI (electromagnetic interference), and the like. As with synchronous noise sources, asynchronous noise sources may also be coupled to the resistive layer.
Ideally, the digitizing panel should amplify the very small corner signal component (of a known frequency) attributable to an object that is in proximity to the digitizing panel, and should reject all other signals (such as the above-mentioned synchronous and asynchronous noise sources) outside a given bandwidth. The digitizing panel should also measure the amplitude of the corner signal with a high degree of precision in order to accurately determine the position of the object relative to the digitizing panel.
However, when amplifying very small signals in a noisy environment, large impulse-type noise sources may cause a high gain amplifier stage to enter a non-linear state thus affecting the ability of the digitizing panel to accurately determine the position of the object relative to the digitizing panel. This is because impulse-type noise may be several orders of magnitude larger than the intended input signal. The slow recovery time of the amplifier stage further increases the detrimental effect of noise impulses by extending the duration of the noise event beyond the time when the noise event is over.
What is needed therefore is an electrostatic digitizing panel that reduces, compensates for, or otherwise filters out, external noise signal components which would otherwise detrimentally affect a reported position of an object.